I think I figured out why.....today I went to look at a 71 swinger and I went to a friend's house who has a 72 and to my surprise I found today that the bumper guards are different from 70/1 and 72 and they mount in different locations. the 72 bumper guard is bigger, fatter on the bottom section and taller. and they mount to line up with inner edge of the turn signal where as 71 lines up dead center to the turn signal lens. so it appears 72 can use the existing bumper bolt hole but in 70/1 the bumper guard mount asymmetrically between the two smooth bolts. so this means a 70/1 had two different bumpers one for guards and one without and needs to drilled to add them to one that did not have previously and a 72 is either or. this makes sense in 72 since that year they started their standardization movement to make less parts (look at the side marker lights as an example) I am starting to think that 72 bumper guards are one year only because they are also dramatically different from 73+
